х+2/9=5/7
x=5/7-2/9
x=(45-14)/63
x=31/63
--------------
х-2/3=3/5
x=3/5+2/3
x=(9+10)/15
x=19/15
x=1 4/15
---------------
5/8-х=1/4
x=5/8-1/4
x=5/8-2/8
x=3/8
----------------
4/5-х=2/7
x=4/5-2/7
x=(28-10)/35
x=18/35
------------------------
2/3+х=2/7
x=2/7-2/3
x=(6-14)/21
x=-8/21
--------------------
1/2+х=2/7
x=2/7-1/2
x=(4-7)/14
x=-3/14
----------------------